Experiments will be conducted to determine the effects of exhaustive exercise on mitochondrial and sarcoplasmic reticulum function and structure of skeletal muscle. Thoroughbred horses and rate will be exposed to a variety of exercise durations and intensities until fatigue. Samples will be collected from trained and non-trained animals in association with the exercise. Horses will be used for studies where the exercise intensity is varied as a function of the individual's maximal oxygen uptake. This will be done on a high speed equine treadmill which permits regulation of the intensity of the exercise and metabolic response of the animals. They will be exercised at submaximal, maximal supramaximal exercise intensities, Horses will be trained for 12 weeks exercising up to 30 miles per week on a half mile training track. Rats will be used to study the effects of long term exercise on the different types of muscle fibers. These animals are ideal for such studies as they have distinctive muscle regions composed almost exclusively of a single fiber type. Rats will be endurance trained for up to 26 weeks with a strenuous program involving running twice per day for up to 3 hr per day. In addition to the studies conducted to determine the mechanisms by which the exercise may induce changes, the potential for training to ameliorate these effects will also be assessed. Studies will involve evaluation of mitochondrial respiratory rate and capacity, lipid oxidation and oxidation of sulfhydryl groups in the sarcoplasmic reticulum changes in phosphoprotein formation and ATP binding in the sarcoplasmic ATPase, mitochondrial hexokinase binding, alterations in the activities of enzymes reducing oxygen free radical formation, and changes in the concentrations of tissue metabolites that may influence mitochondrial respiratory capacities. The time course for recovery of exercise-induced effects will be determined.